A growing number of Northern Ireland teachers fear energy drinks are increasingly contributing to bad behaviour in the classroom, according to a leading trade union official.

As parents struggle to keep packed lunches appetising at the start of a new term, Avril Hall Callaghan, general secretary of the Ulster Teachers' Union, Northern Ireland's biggest locally-based teaching union, warned of the dangers of energy drinks, which can contain as much caffeine as three espresso coffees.

"The seriousness of their abuse was highlighted this summer in the tragic case of a mum-of-three from Northern Ireland who is going blind because of her apparent addiction to energy drinks - consuming 28 tins of Red Bull daily," she said.

"What we are hearing from members is that many believe these drinks - which have been labelled as another form of 'legal high' - are contributing to behavioural problems in class.

"The problem arises when young people are using the drinks to enable them to stay up until the early hours of the morning and then drink two or three cans filled with sugar and caffeine on the way to school to make up for their lack of sleep."

The teachers' leader said that students arrive in class hyperactive and unable to concentrate, and then experience a "sugar crash" later in the school day when the impact of the drinks wears off.

"Pupils' behaviour, concentration and energy all suffer," she said.

"People do not realise how harmful caffeine can be.

"However, parents can play their part in correcting this trend by encouraging their children to eat and drink healthy lunches, and by under-pinning the work of schools in promoting healthy lifestyles at home."

The problem is apparent throughout the post-primary school system, according to UTU deputy general secretary Jacqui Reid. "The way these drinks are marketed lead children to think they are very cool, but the drinks leave the youngsters uptight, anxious, jumpy and irritable," she said.

"Teenagers are already going through a lot of stress and these energy drinks just add an extra layer of stress on top of it."

Newtownabbey mum Lena Lupari ballooned to 26 stone after spending almost £6,000 every year on the high-calorie energy drink.

Doctors warned Ms Lupari to lose seven stone in order to keep her medical problems - including idiopathic intracranial hypertension, which can lead to vision loss - at bay.

A spokeswoman for Red Bull said the drink should be "consumed in moderation".

"Like any foods or beverages, energy drinks should be consumed in moderation and as part of a balanced and varied diet and healthy lifestyle," the spokeswoman said.

"Our product is predominantly sold in small cans, which contributes to such moderation."